G-protein coupled receptors play important roles in diverse signaling processes, including those involved in host defense mechanisms. Immune responses to infectious diseases, injury, tumors and organ transplantation and in diseases and conditions such as asthma, allergy, rheumatoid arthritis and neoplasia have been linked to GPCR regulation. Exaggerated or misdirected immune responses are responsible for many inflammatory and hypersensitivity diseases which, left untreated, can result in tissue or organ damage, pain and/or loss of function. Tissue inflammation is largely implicated in the pathogenesis of such diseases, of which asthma and allergic diseases are among the most well characterized. The mechanisms underlying airway inflammation and hyperreactivity are similar to those underlying allergic inflammation in other tissues, such as the skin and gut.
Prostaglandins are lipid-derived inflammatory mediators that recruit macrophages, T cells, eosinophils, basophils and neutrophils from peripheral blood to damaged or inflamed tissues. In addition, prostaglandins can, depending on the target cell type, induce or inhibit intracellular Ca2+ mobilization, cAMP production, platelet aggregation, leukocyte aggregation, T cell proliferation, lymphocyte migration, and Th2 cell chemotaxis, IL-1a and IL-2 secretion and vascular and non-vascular smooth muscle contraction in responsive cells. Prostaglandins have been implicated in fever, various allergic diseases, vascular and non-vascular smooth muscle relaxation, pain perception, sleep, platelet aggregation and reproductive processes. Prostaglandins exert their effects by interacting with specific GPCRs.
Prostaglandin D2 (PGD2) is the major inflammatory mediator released by activated mast cells, typically found near skin surfaces, mucous membranes and blood vessels, upon immunological challenge (Lewis et al. (1982) J. Immunol. 129:1627-1631). During asthma and allergic responses, PGD2 is released in large amounts. The role of PGD2 in the initiation and maintenance of allergic inflammation has been well established in mouse models of asthma. For example, it has been demonstrated that overproduction of PGD2 in vivo by PGD2 synthase exacerbates airway inflammation in a mouse model of asthma (Fujitani et al. (2002) J. Immunol. 168:443-449).
A PGD2-selective receptor, designated DP, has been identified (Boie et al. (1995) J. Biol. Chem. 270:18910-18916). In humans, DP is expressed in smooth muscle, platelets, small intestine and brain, and its expression in lung epithelium is induced by allergic challenge. Receptor activation induces cAMP production and intracellular Ca2+ mobilization, and is believed to inhibit platelet aggregation and cell migration and induce relaxation of various smooth muscles. DP is coupled primarily to Gαs protein.
Significantly, in an OVA induced asthma model, DP−/− mice exhibited reduced asthma symptoms, e.g., reduced cellular infiltration of eosinophils and lymphocytes in BAL fluid, reduced Th2 cytokine levels in BAL fluid and reduced airway hyperreactivity to acetylcholine (Matsuoka et al. (2002) Science 287:2013-2019). The increased cellular infiltration in lung tissue and mucus secretion by airway epithelial cells characteristic of asthma in humans and observed in wild-type mice was not observed in DP-deficient mice.
Recently, an additional PGD2-selective receptor, designated chemoattractant receptor-homologous molecule expressed on Th2 cells, or CRTH2, has been identified (Hirai et al. (2001) J. Exp. Med. 193(2):255-261). The receptor was previously referred to as GPR44 or DL1R. Among peripheral blood T lymphocytes, human CRTH2 is selectively expressed on Th2 cells, and is highly expressed on cell types associated with allergic inflammation such as eosinophils, basophils and Th2 cells. It has been shown that CRTH2 activation induces intracellular Ca2+ mobilization and infiltration of Th2 cells, eosinophils and basophils.
Protein sequence analysis indicates that CRTH2 has no significant homology to DP, but rather, is related to members of the N-formyl peptide receptor (FPR) subfamily (Nagata et al. (1999) J. Immunol. 162:1278-1286). In contrast to DP, CRTH2 has been shown to couple primarily to Gαi protein.
These observations suggest that CRTH2 and DP may function independently to regulate aspects of allergic inflammation.
The increasing incidence of asthma, allergic diseases and immunologic diseases worldwide underscores the need for new therapies to effectively treat or prevent these diseases. The discovery of small molecules that modulate CRTH2 and/or one or more other PGD2 receptors, e.g., DP, is useful for the study of physiological processes mediated by CRTH2 and/or one or more other PGD2 receptors, e.g., DP, and the development of therapeutic agents for asthma, allergic diseases and other immunologic diseases. Novel compounds which display such desirable activity are described herein.